1. Field of the Invention
The present invention is generally directed to vehicle suspension systems, and in particular, to control arrangement for such suspension systems. The present invention will be described in relation to the Applicant""s vehicle suspension systems. It is however envisaged the present invention also has general application in vehicle suspension systems.
2. Description of the Related Art
The applicant has developed vehicle suspension systems which utilize lateral stabilizer bars which can be used in conjunction with fluid circuits to thereby provide roll stabilization while facilitating free cross-axle articulation for the suspension system (the cross-axle articulation mode in a common direction with respect to the body, and the other pair of wheels move in the opposite direction. This mode of wheel motion is commonly referred to as warp). Suspension system of this type are described in U.S. Pat. Nos. 6,302,417 and 6,217,047, details of which are incorporated herein by reference. The suspension systems have the advantage of being passive systems which do not require an external energy source to operate properly. By comparison, active suspension systems typically require at least a fluid pump to supply fluid to actuators controlling the wheel motions.
There are however circumstances which make it advantageous to provide a control arrangement to adjust the volume of fluid within the fluid circuits of the Applicant""s vehicle suspension systems.
The volume of fluid in the fluid circuits may need to be adjusted for at least the following reasons:
1) To permit levelling of the roll attitude of the vehicle body with respect to the wheels without introducing a bias in one direction in the fluid circuit; and
2) To prevent fluid expansion caused by temperature rises causing over pressure in the fluid circuits. This can be a problem since there is little resilience in the conduits of the fluid circuits and accumulators are normally not provided in the fluid circuits. The addition of accumulators can detract from the roll control of the vehicle where they are provided to accommodate fluid expansion within the fluid circuit because this can result in variation of the roll rates of the vehicle.
The extent of this temperature related problem, can be understood from the following case. When the vehicle is started in the city the system can be set with the correct volume of fluid for the essentially cold start. As it is run at low speed or under stop-start conditions within the city, the temperature in the fluid circuit rises resulting in an increase in the fluid volume therein. It is therefore preferred in this situation, that fluid be released from the circuit. Leaving the city, speeds increase, providing increased air flow over the components of the fluid circuit thereby cooling them down. The fluid then contracts which can leave the suspension system with reduced roll control since any small air bubbles in the fluid circuit can expand resulting in an initial additional compliance about the roll axis of the vehicle. To keep the roll rate constant, the pressure of the fluid in the roll system therefore preferably needs to be kept at an average design pressure.
It is therefore an object of the present invention to provide a control arrangement for controlling the fluid volume and therefore the pressure within the fluid circuit of a vehicle suspension system.
With this in mind, there is provided a vehicle suspension control system for a vehicle,
the vehicle having at least one forward pair of wheels and at least one rearward pair of wheels connected to the body of the vehicle to allow substantially vertical relative motion of each wheel with respect to the body,
the suspension system of the vehicle including front and rear resilient support means for supporting the vehicle body with respect to the wheels, a front roll stabilisation assembly interconnecting at least one said forward pair of transversely spaced wheels and a rear roll stabilisation assembly interconnecting at least one said rearward pair of transversely spaced wheels, each roll stabilisation assembly including at least one lateral torsion bar and a double-acting hydraulic actuator interconnected to the at least one lateral torsion bar, the front and rear hydraulic actuators being interconnected by first and second fluid conduits such that
roll moments applied to the vehicle body generate pressure within the fluid conduits thereby transmitting the roll moment into each lateral torsion bar to react against at least a portion of said roll moment, and
warp motions of the wheels with respect to the vehicle body generate flow along the fluid conduits resulting in a displacement of one said hydraulic actuator in a proportional and opposite direction to the other said hydraulic actuator,
the front and rear roll stabiliser assemblies thereby providing roll stiffness during both roll and warp motions of the wheels with respect to the body while at the same time providing substantially zero warp stiffness,
wherein the control system includes an hydraulic fluid supply means and fluid conduit valve means for selectively communicating the fluid conduits with the hydraulic fluid supply means to thereby regulate the average pressure in both the first and second fluid conduits.
This control system does not require a fluid pump as fluid is vented from the fluid conduits to the reservoir, or is returned to the fluid conduits from the tank by controlling the valves in each fluid line.
A flow restricting means may be provided for each valve means for preventing rapid discharge of fluid from the fluid conduits.
The valve means may be in the form of an electric solenoid valve having xe2x80x9cflow onxe2x80x9d and xe2x80x9cflow offxe2x80x9d positions. The valve may normally be in the flow off position and may be moved to the flow on position when venting fluid from the fluid circuit or when supplying fluid to the fluid circuit is required.
Non-return valve means may optionally be provided in a bypass line bypassing the valve means. The non-return valve means allows the ingress of fluid when the pressure in the fluid circuit falls below a design pressure, for example when there is a drop in temperature in the fluid circuit.
Accumulators may also optionally be provided on each fluid conduit for providing a degree of resilience in the operation of the fluid circuit. This can provide a reduced roll stiffness zone around the level position and reduce the effect of temperature changes on the roll system pressure.
It should be noted that empirical testing has shown that the design pressure of the fluid circuit can be atmospheric (plus whatever head is provided by the position of the fluid tank). This can be desirable for reasons of cost, complexity and reliability. By keeping the design pressures of the hydraulic roll control system low when reacting roll moments on the vehicle body, the amount of additional resilience introduced by the expansion of the flexible hydraulic hoses in roll can be reduced.
Pressure sensing means may be provided for measuring pressure changes in the fluid circuit. The pressure sensing means may be located on the fluid conduits or on the fluid line communicating the valve means with the fluid conduits.
Releasing fluid can be controlled hydraulically or electronically. The electronic option is preferred due to the increased intelligence which can be built in to prevent too much fluid being released from one fluid conduit (the pressure change in the fluid circuit due to temperature effects being determined at least in part by the total volume of fluid in the fluid circuit). For example, to reduce pressure and maintain body roll attitude, fluid should preferably be released from both conduits in the correct proportions. Releasing fluid from just one conduit will reduce pressure, but may also cause the body roll attitude to change. This can be used intelligently to correct for roll attitude errors (although the fluid volume changes due to temperature are small). The method of control for the release of fluid may be pulsed valving with monitoring of body attitude.
According to another aspect of the present invention, there is provided a method of controlling a vehicle suspension system for a vehicle having at least one forward pair of wheels and at least one rearward pair of wheels connected to the body of the vehicle to allow substantially vertical relative motion of each wheel with respect to the body,
the suspension system of the vehicle including front and rear resilient support means for supporting the vehicle body with respect to the wheels, a front roll stabilisation assembly interconnecting at least one said forward pair of transversely spaced wheels and a rear roll stabilisation assembly interconnecting at least one said rearward pair of transversely spaced wheels, each roll stabilisation assembly including at least one lateral torsion bar and a double-acting hydraulic actuator interconnected to the at least one lateral torsion bar, the front and rear hydraulic actuators being interconnected by first and second fluid conduits such that
roll moments applied to the vehicle body generate pressure within the fluid conduits thereby transmitting the roll moment into each lateral torsion bar to react against at least a portion of said roll moment, and
warp motions of the wheels with respect to the vehicle body generate flow along the fluid conduits resulting in a displacement of one said hydraulic actuator in a proportional and opposite direction to the other said hydraulic actuator,
the front and rear roll stabiliser assemblies thereby providing roll stiffness during both roll and warp motions of the wheels with respect to the body while at the same time providing substantially zero warp stiffness,
the method including selectively communicating the fluid conduits with a hydraulic fluid supply means to thereby regulate the average pressure in both the first and second fluid conduits.
Introducing fluid into the fluid circuit may preferably be achieved by supplementing the volume due to contraction or to level the vehicle. In suspension systems described in International Application No. PCT/AU96/00528, for example in the embodiment shown in FIG. 12 of this application, fluid can simply be supplied to the fluid conduits by opening the valves to allow fluid flow from the fluid tank to the conduits. This is because the coil spring support means supporting the vehicle weight also provide a degree of roll control.
However, in the case of the suspension systems described in U.S. Pat. No. 6,217,047, where the support means supporting the weight of the vehicle may provide substantially no roll control, an alternative procedure is required. Referring to FIG. 4 of said application, which shows a vehicle suspension system using laterally connected air springs to support the vehicle weight, this may require temporarily blocking off of the interconnection between the air springs to provide a degree of roll stiffness for the air springs. The fluid circuit can then be vented to the tankxe2x80x94detection of cornering preferably being provided to ensure that the vehicle is not left with reduced roll stiffness at an inopportune time. The body can then be levelled on the air spring support system using any known trim method and the fluid circuit then resealed. With the fluid circuit once again providing roll stiffness, the air spring interconnections can then be reopened.
Using the roll system to react any eccentric loads on the body can produce an uneven roll stiffness left to right, especially if the roll movement generated by the load is large and if the roll system is provided with a reduced stiffness zone about the level roll attitude position for comfort (as provided by the optional accumulators). The roll system may still be periodically vented to maintain a substantially constant working pressure when the vehicle is running in a straight line.
Therefore another arrangement of the levelling system may be applied to a vehicle suspension system having front and rear stabiliser bars interconnected by an hydraulic interconnection and supports which are laterally interconnected at one end of the vehicle and independent at the other. The independent pair of supports may be used to level the roll attitude of the vehicle in addition to providing height control of the associated end of the vehicle body. The interconnected supports providing control of the height of the other end of the vehicle body. Wheel position sensors may be used to monitor the position of each wheel to determine the height and roll attitude of the vehicle body in relation to the average ground plane and also the degree of the warp of the ground plane. A controller may use these wheel position inputs to calculate the need for any change in the volume of the fluid in the support means. The controller may also have a height selector input such that the driver may select between different heights of the vehicle body. These height inputs may be overridden by the controller in dependence, for example, on an input relating to the speed of the vehicle. The controller may also have different modes of adjustment each having either different speeds of operation, wheel position sensor sampling times and speeds, control valve opening times and/or wheel position tolerances in the acceptable set up. The operation between these modes may be determined by inputs such as vehicle speed, wheel positions being within or outside tolerance bands and ignition on or off signals. One mode may be operational for preset time after the ignition has been switched off.
If periodical venting of the roll system is used, this venting may be subject to a signal indicating a lateral acceleration below a preset level. The system may also be vented before, during or after a support system levelling or height change operations to ensure that any bias in the roll system is removed and does not prevent the correct set up of the system. The venting system may include an hydraulic tank which the roll system circuits are vented to. The tank may be pressurised using the fluid pressure source from the support system.
The detection of cornering can be done by any known means although a lateral acceleration sensor may preferably be provided to give the input to a variable roll moment distribution/wheel lift lockout controller. If a cornering detection device or method is used, it may be preferable to prevent any levelling operations when cornering is detected.
If a roll system is provided with the optional accumulators, the need to control the fluid volume within the circuits with pressure change is negated by the ability of the accumulators to absorb a small increase in fluid volume with little resultant pressure change. The roll system can therefore be sealed if there is provided at least one accumulator for each roll circuit. To reduce the delay in turn-in response of a roll system with additional resilience provided by hydropneumatic accumulators, the accumulators may be locked off from the roll circuits when cornering is detected. This allows at least two different roll stiffness rates to be provided by the system, a high roll stiffness to limit roll in corners and a reduced roll stiffness for most other situations which increases comfort. In order to speed the response time of the control system in detecting cornering, a simple steering angle input (or a function of steering angle and speed for example) may be used to initially trigger the locking off of the accumulators. However, if through this simple control the accumulators are locked off needlessly, a check may be done, based on a function of for example, the steering angle, rate of change of steering angle, vehicle speed and the lateral acceleration and/or yaw of the vehicle body. The accumulators may be connected to the roll circuits via simple multi-stage dampers and lockout valves and/or variable restrictors. If roll moment distribution changing lockouts are provided, there must be at least one accumulator for both parts of both roll circuits. Alternatively, since the roll moment distribution changing lockouts and the accumulator lockouts have similar control inputs, they may be activated by a single controller. This may negate the need for more than one accumulator in the upper roll circuit and one in the lower roll circuit.
It will be convenient to further describe the invention by reference to the accompanying drawings which illustrate possible embodiments of the present invention. Other embodiments of the invention are possible and consequently the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. Common reference numerals are used for similar parts between drawings.